Hi Ian,
Classic pattern refers to pattern as it was flown prior to the all turnaround era.
There are many differences in classic pattern from "normal (AMA) pattern. The primary difference is that there is no box. The pattern is not flown as a sequence, I.E. "turnaround" style, but rather one maneuver is flown per pass past center. Whatever the pilot does to reverse course for the next maneuver is up to the pilot, and is not judged. As far as manueveras are concerned, some are the same (loops, immelman turns, stall turn, etc) while some types of manuevers flown in AMA pattern are not part of classic (shark's tooth, humpty bump). Here is a link to the rules currently in use by the Classic PAttern Association:
http://www.classicpatternassociation...formation.html
While you are on the CPA website, check out the exciting video on the home page!

The second major variation between the two styles of pattern are the type of airplanes that are flown. Classic pattern is flown primarily using airplanes designed prior to the 2 meter rule, which is when AMA pattern abandoned one maneuver per pass flying in favor of all turnaround format. most popular are those designs dating fron the 1970's to the mid eighties.

Here is a good FAQ sheet that will explain further.
http://www.necpo.org/faq.php

-Robert